Restricted Earth Fault (REF) protection is a type of protection used in electrical distribution systems to detect small leaks of current to earth. Some key points about REF protection:
- It is used where there is a requirement to quickly detect earth faults with small fault currents, typically less than 25% of the line current. This helps avoid unnecessary trips for minor earth faults.
- REF relays operate by measuring the imbalance between the line currents in a multi-conductor system, rather than directly measuring earth fault current. This allows them to detect very small earth leaks.
- Typical applications include medium voltage industrial power systems, mining equipment power supplies, steel mills, and railway electrical distribution systems.
- The REF relay is connected to a current transformer that links all line conductors of a circuit. It sums the line currents and looks for any imbalance, indicating a leak of current to earth on one of the phases.
- REF provides increased sensitivity over traditional earth fault protection methods, allowing earth faults to be cleared more rapidly while avoiding unnecessary trips on small transient earth faults.
- Drawbacks can include increased complexity and cost compared to simple earth fault protection. Correct CT connections and settings are also important for proper operation.
The main differences between restricted earth fault (REF) protection and traditional earth fault protection methods are:
- Measurement technique – Traditional methods use a neutral/earth current transformer to directly measure any earth fault current. REF instead sums the line currents and looks for imbalances, allowing it to detect much smaller earth leaks.
- Sensitivity – REF can detect earth faults down to around 20-25% of line current, compared to 50-100% for traditional overcurrent-based methods. This provides faster operation for minor faults.
- Fault indication – Traditional methods only indicate an earth fault is occurring but not its location. REF can locate the faulty phase by sensing which line current is imbalanced.
- Application – REF is typically used where very low-level earth faults need rapid clearance, such as industrial systems. Traditional methods are adequate for general power distribution systems.
- Neutral connection – Traditional methods often rely on a solidly earthed neutral connection for operation. REF does not require a dedicated neutral and can be used with unearthed or noon-effectively earthed systems.
- Complexity – Traditional earth fault relays are simpler bypass types. REF requires additional current transformers, wiring and processing to sum and compare line currents.
- Cost – Traditional methods use standard protection relays and CTs. REF requires matching multiphase CTs and more advanced microprocessor relays, making it more expensive.